This invention relates to medical devices for performing diagnostic, mapping, ablation, and other procedures and, more particularly, to a medical device for incrementally moving an electrode a predetermined distance.
Cardiac arrhythmias (commonly known as irregular heart beats or racing hearts) are the result of various physical defects in the heart itself. One such defect comprises an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses normally existing in the heart. This accessory pathway often causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber, causing the heart to beat irregularly and therefore inefficiently pump blood.
Another common type of cardiac arrhythmia is ventricular tachycardia (VT), which may be a complication resulting from a heart attack or from a temporary reduction of blood supply to an area of heart muscle. VT is often caused by a tiny lesion, typically on the order of one to two millimeters, that is located close to the inner surface of the heart chamber. That lesion is often referred to as an xe2x80x9cactive sitexe2x80x9d, because it does not fire in sequence with the rest of the heart muscle. VT causes the heart""s normal rhythmic contraction to be altered, thereby affecting heart function. A typical symptom is rapid, inefficient heart beats.
Other common cardiac arrhythmias include atrial flutter and atrial fibrillation, which originate in the atria and cause the atria to beat so rapidly that they quiver (i.e., fibrillate). This in turn causes the ventricles to beat too fast (up to 200 beats per minute), which results in an inefficient pumping of blood.
Non-surgical procedures, such as management with drugs, have been proposed for treating cardiac arrhythmias. However, some arrhythmias are not treatable with drugs. For example, drug therapy to combat VT is typically successful in only 30 to 50 percent of patients. Because of this low success rate, another conventional remedy is to perform a surgical procedure in which various incisions are made in the heart to block conduction pathways, and thereby divide the atrial area available for multiple wavelet reentry in an effort to abolish the arrhythmia. Alternatively, an automatic implantable cardioverter/defibrillator (AICD) can be surgically implanted into the patient, as described in U.S. Pat. No. 4,817,608 to Shapland et al. While these surgical procedures can be curative, they are associated with increased morbidity and mortality rates, and are extremely expensive. Even the use of an AICD requires major surgical intervention. Moreover, patients of advanced age or illness often cannot tolerate invasive surgery to excise the tachycardia focus which causes the arrhythmia. Thus, this type of treatment is unavailable to many.
Non-surgical, minimally invasive techniques have been developed which are used to locate cardiac regions responsible for the cardiac arrhythmia, and to disable the short-circuit function of these areas. According to these techniques, electrical energy shocks are applied to a portion of the heart tissue to ablate that tissue and produce scars which interrupt the reentrant conduction pathways. The regions to be ablated are usually first determined by endocardial mapping techniques. Mapping typically involves percutaneously introducing a diagnostic catheter, having one or more electrodes, into the patient, passing the diagnostic catheter through a blood vessel (e.g., the femoral vein or aorta) and into an endocardial site (e.g., the atrium or ventricle of the heart), and inducing a tachycardia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardial positions. When a tachycardia focus is located, as indicated in the electrocardiogram recording, it is marked by means of a fluoroscopic image so that the site can be ablated. A conventional electrode catheter, having electrodes with a greater surface area than the diagnostic catheter""s electrodes, can then provide electrical energy to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will create a region of necrotic tissue to disable the malfunction caused by the tachycardia focus.
Conventional catheter ablation techniques have used catheters each having a single electrode fitted at its tip as one electrical pole. The other electrical pole is conventionally provided by a backplate in contact with a patient""s external body part to form a capacitive coupling of the ablation energy source (DC, laser, RF, etc.). Other ablation catheters are known in which multiple electrodes are provided.
Ablation is carried out by applying energy to the catheter electrodes once the electrodes are in contact with the cardiac tissue. The energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. When RF energy is delivered between the distal tip of a standard electrode catheter and a backplate, there is a localized RF heating effect. This creates a well-defined, discrete lesion slightly larger than the tip electrode (i.e., the xe2x80x9cdamage rangexe2x80x9d for the electrode), and also causes the temperature of the tissue in contact with the electrode to rise.
To overcome certain types of cardiac arrhythmia, such as atrial flutter and atrial fibrillation, it is often necessary to create a long, continuous lesion (i.e., a linear lesion) to block the aberrant pathway(s). One conventional ablation procedure for creating linear lesions is commonly referred to as a xe2x80x9cdragxe2x80x9d method. According to that method, an ablation catheter carrying one or more ablation electrodes is manipulated through a patient""s blood vessels to a desired location within the patient""s heart. One or more of the electrodes is manipulated into contact with the heart tissue. Ablation energy is then delivered to the electrode(s), causing them to heat up and scar the adjacent tissue to create a lesion which is typically slightly larger than the surface area of the electrode contacting the tissue (the electrode""s damage range). After the electrode has been disposed in that location for a sufficient time to ablate the adjacent tissue, the clinician then manually moves the catheter a selected amount by pulling on the catheter shaft, and ablation energy is again delivered to the electrode(s) to ablate the tissue that is then adjacent to the electrode. By continuing this procedure, the clinician attempts to create a continuous, linear lesion to block an aberrant pathway.
However, to create a continuous lesion, the clinician must be careful not to move the catheter too far between successive ablations. If the clinician should accidentally move the catheter too far, then the lesion created will not be continuous, and the aberrant pathway may not be destroyed, requiring that the patient undergo yet another procedure, which is inefficient and undesirable.
Accordingly, it will be apparent that there continues to be a need for a device for performing ablations which ensures the creation of linear lesions, by automatically displacing an ablation electrode in successive, incremental movements of a predetermined distance. In addition, the need exists for a device which moves an electrode in known increments to perform other medical procedures. The instant invention addresses these needs.
According to one aspect of the invention, an electrode is connected to a movable member, such as a catheter shaft or an outer sheath, which is slidably extended over a guide wire, flexible shaft, or other tubular member. A displacement mechanism is connected to the movable member, and may be actuated one or more times to displace the movable member in successive, predetermined increments. In this manner, the electrode is reliably moved in constant increments, and is suitable for creating a linear lesion or for performing diagnostic functions, without forcing the clinician to estimate the distance the electrode has been moved.
Thus, in one illustrative embodiment, the present invention is directed to a medical device comprising an elongated shaft, an electrode mounted on the shaft, and an electrode displacement mechanism connected to the shaft and operative to displace the shaft in predetermined increments.
In another illustrative embodiment, the invention is directed to a method for creating continuous lesions, comprising: (a) positioning an ablation electrode at a selected site within a patient, the ablation electrode having predetermined dimensions; (b) delivering ablation energy to the electrode to ablate the patient""s tissue disposed adjacent to the tissue; (c) displacing the electrode in a predetermined increment, wherein the predetermined increment is determined based upon one or more of the dimensions of the electrode; and (d) repeating steps (b) and (c) one or more times to create a continuous lesion.